Resolves the argument to a numeric constant, which can be passed to
the wrap_socket function/method from the ssl module.
Defaults to ssl.CERT_NONE.
If given a string it is assumed to be the name of the constant in the
ssl module or its abbrevation.
(So you can specify REQUIRED instead of CERT_REQUIRED.
If it’s neither None nor a string we assume it is already the numeric
constant which can directly be passed to wrap_socket.